Organic Chemistry Reasoning Questions and Answers

Q. Tell
which is the stronger base and why.

Ans

(a) Aniline or o-bromoaniline.
Aniline is a stronger base than o-bromoaniline. Aniline
form anilinium ion by accepting a proton.

In general, electron-donating groups such as -CH3,
OCH3, -NH2 etc. increase the
basecity by stabilizing the anilinium ion while electron
withdrawing substituents such as - NO2, -CN,
-X (halogens) decrease the basicity of amines by
destabilizing the anilinium ion. But o-substituted
anilines are usually weaker bases than anilines
regardless of the nature of the sulistituent (electron
withdrawing or donating). This is due to Detho-effect
which is due to combination of steric and electronic
factors.
In case of halogens (-Cl, -Br, -I), electron withdrawing
inductive effect dominates so it is expected that
o-bromoaniline should be weaker base than aniline

(b) aniline or cyclohexylamine
Cyclohexylamine is a stronger base than aniline. This is
because lone pair of electrons on the N- atom of aniline
is declocalized over the benzene ring.

However in cyclhexylamine the lone pair
of electrons on N- atom are not conjugated and are
therefore more readily available for donation or
protonation than that on the N- atom of aniline. Further
N- atom inaniline is sp2 hybridized and in
cyclohexylamine it is wp3 hybridized. Since sp2
has greater electronegative character than sp3,
the electrons are more tightly held in case of aniline
whereas they are losely held and readily available for
protonation in cyclohexylamine.

Q.
Illustrate the amphoteric nature of amino acids by
writing an equation for the reaction of proline in its
dipolar ion form with an equivalent of

Ans

(a) hydrochloric acid. HCl

(b) sodium hydroxide, NaOH

In general, in dipolan ion form
amino acid can accept a proton from acid and exist as
cation and can donate a proton to base and exist as
anion.
This shows its amphoteric behaviour.